The present invention relates to stents having a radial stiffness that varies along the length of the stent. Specifically, the invention relates to a stent that actively changes the density of rings in different zones along the axial length of the stent when the stent is expanded from a compressed condition to an expanded condition.
Surgical stents have long been known which can be surgically implanted into a body lumen, such as an artery, to reinforce, support, repair or otherwise enhance the performance of the lumen. For example, in cardiovascular surgery it is often desirable to place a stent in the coronary artery at a location where the artery is damaged or is susceptible to collapse. The stent, once in place, reinforces that portion of the artery allowing normal blood flow to occur through the artery. One form of stent which is particularly desirable for implantation in arteries and other body lumens is a cylindrical stent which can be radially expanded from a first smaller diameter to a second larger diameter. Such radially expandable stents can be inserted into the artery by being located on a catheter and introduced internally through the arterial pathways of the patient until the unexpanded stent is located where desired. The catheter is fitted with a balloon or other expansion mechanism which exerts a radial pressure outward on the stent causing the stent to expand radially to a larger diameter. Such expandable stents exhibit sufficient rigidity after being expanded that they will remain expanded after the catheter has been removed. An example of a stent known in the art is shown in FIG. 4 and FIG. 5. With initial reference to FIG. 4, the stent 50 shown there generally comprises a plurality of radially expandable cylindrical elements or rings 52 disposed generally coaxially along the axis 2 of the stent and interconnected by elements 54, 56 disposed between adjacent cylindrical elements. While FIG. 4 shows the stent 50 in a compressed or unexpanded condition suitable for delivery into the vasculature of a patient, FIG. 5 shows the same stent 50 in an expanded condition after placement and expansion in the vasculature.
Yet, known prior art stents suffer from a variety of drawbacks when certain types of deficiency or damage in an artery is encountered by a surgeon. For example, a surgeon may encounter an arterial situation where a stent is required to have greater radial support stiffness at the ends of the stent, and less radial support stiffness in the middle between the two ends. Another situation may be encountered requiring a stent having a greater radial support stiffness in the middle, and less radial support stiffness at the ends.
Certain stents have been developed to satisfy these requirements, but they suffer from drawbacks. For example, one approach describes a stent with greater radial support stiffness in the middle of the stent, wherein the struts are made thicker in the middle of the stent and thinner towards the ends. Another approach describes a stent with varying radial strength, wherein the desired effect is accomplished by increasing the width of the struts, or increasing the length of a cylindrical element. Yet another approach describes a stent in which the structural members are provided with regions having different widths, and tapering widths of selected segments. However, these solutions may result in a stent that is expensive to manufacture, and has non-uniform bending characteristics about the longitudinal axis and this latter aspect may introduce complications during delivery. Yet another approach describes a stent of differentiated stiffness that is achieved with the use of a superelastic material that transitions between a relatively soft and malleable phase to a stiffer phase at a transition temperature that is adjustable along the length of the stent. By differentially adjusting the transition temperature of different portions of the stent, a differentiation in the stiffness of the structure is achieved upon the stent being subjected to body temperature. However, this solution entails complex differentiated metallurgical application in a single stent in order to achieve the desired result.
Thus there is a need in the art for a simple and effective stent that possesses varying degrees of radial support stiffness over its length, that is easy to manufacture, and has substantially uniform longitudinal flexibility. The present invention addresses these and other needs.